1. Field of the Invention
The present invention relates to an optical scanner for forming an electrostatic latent image on a photosensitive body by scanning the body with a laser beam. The invention also relates to an image forming apparatus fitted with such a scanner.
2. Description of the Related Art
A conventional image forming apparatus such as a laser printer is fitted with an optical scanner for forming an electrostatic latent image on a photosensitive body by scanning the body with a laser beam. This scanner includes a scanning optical system consisting mainly of a semiconductor laser as a light source, a galvanomirror, and a scanning lens.
The galvanomirror is supported rotatably on a shaft and turns on it within a specified scanning range. While the galvanomirror is turning on the shaft at a uniform angular velocity, the galvanomirror scans the photosensitive body along primary scanning lines by reflecting the laser beam emitted from the semiconductor laser. The direction in which the galvanomirror turns on the shaft changes with the polarity of the magnetic field generated by the inductor of the optical scanner.
If the galvanomirror scanned the photosensitive drum directly without a scanning lens, a difference would arise in beam-condensing timing between the middle and each end of the drum. This would make it impossible to obtain an image of uniform quality. Therefore, the optical scanner of the conventional image forming apparatus is fitted with an fθ lens or another scanning lens for so correcting the difference in beam-condensing timing that the reflected beam describes a linear locus.
The optical scanner of the conventional image forming apparatus forms an electrostatic latent image on the photosensitive body by means of both the primary scanning performed by the reflection of the laser beam from the turning galvanomirror and the secondary scanning performed by the movement of the photosensitive body. If the photosensitive body is a drum, the latent image can be recorded at a higher density by either rotating the drum at a lower speed or turning the galvanomirror at a lower angular velocity. Accordingly, this scanner makes it possible to form an image of higher pixel density by switching both the rotational speed of the photosensitive drum and the angular velocity of the galvanomirror to lower values.
JP H4-314019 A discloses an optical scanner for scanning a photosensitive body with the laser beam reflected by a galvanomirror.
The speed at which the galvanomirror scans the photosensitive body along one of the primary scanning lines may differ from the speed at which the galvanomirror scans the body along another. In many cases, a difference may arise in scanning speed between two primary scanning lines along which the photosensitive body is scanned in opposite directions. Differences in scanning speed may be due to the aged deterioration of the galvanomirror shaft. Differences in scanning speed among the primary scanning lines make differences in pixel density among the lines. This results in the formation of an electrostatic latent image of heterogeneous quality on the photosensitive body. The development of the latent image of heterogeneous quality results in the formation of a microscopically mottled image on a sheet of paper.
Thus, the optical scanner of the conventional image forming apparatus cannot form an image of uniform quality, although it can form an image of high pixel density.
An object of the present invention is to provide an optical scanner for forming an electrostatic latent image of uniform quality on a photosensitive body by scanning the body along primary scanning lines at a controlled constant speed. Another object of the invention is to provide an image forming apparatus fitted with such a scanner.